Various mechanical and electromechanical instruments have been developed for measuring acceleration, inclination, velocity, and motion, including piezoelectric and piezoresistive instruments, and force balanced, capacitive or convective accelerometers.
In a force-balanced accelerometer, an inertial mass is suspended by a spring that allows it to move between two permanent magnets. When a force-balanced accelerometer experiences an external acceleration, the inertial mass is displaced from its normal resting position. A sensor within the accelerometer produces an electrical signal that is subsequently amplified and passed through a conductive coil that surrounds the mass. The level of amplification is selected such that the conductive coil produces a rebalancing force that restores the inertial mass to its original resting position. Because the magnitude of the rebalancing force is proportional to the external acceleration, the magnitude of the external acceleration can be determined by measuring the amplified electrical signal. Accelerometers of this type have high sensitivity and accuracy. However, they are expensive, susceptible to mechanical wear, and only capable of measuring linear accelerations.
An example of an accelerometer capable of measuring angular accelerations is a resistive accelerometer. In this type of accelerometer, gas is injected through a nozzle into a chamber while an external acceleration is applied. The chamber has two wires arranged so that the injected gas is uniformly distributed between the sensing elements in the absence of external acceleration. In the presence of acceleration, the injected gas will tend to accumulate near one of the wires, causing it to become colder than the other wire by convective cooling. In turn, this causes a measurable difference in the resistance of the two sensing elements that is proportional to the angular velocity. However, a significant disadvantage of this type of accelerometer is that it requires the presence of a spraying nozzle, which makes the accelerometer bulky and expensive.
Yet another type of accelerometer is a convective accelerometer. An example of a prior convective accelerometer is one that contains a heating element installed at the center of housing, with two temperature sensing elements arranged symmetrically in the housing with respect to the heating element. The heating element heats a gas enclosed in the housing, causing it to circulate symmetrically about the housing in the absence of an external acceleration. In this situation, the temperature sensors are at the same temperature, so that the difference in their readings is essentially zero, indicating a quiescent state. However, when an external acceleration is applied, the gas no longer circulates symmetrically, which causes the sensing elements to be at different temperatures. The magnitude of the temperature difference is proportional to the external acceleration. However, convective accelerometers of this type have significant disadvantages, including low dynamic range, low sensitivity, inability to measure purely rotational motion, and high energy consumption of energy due to the energy requirements of the heating element.
Another type of accelerometer is a linear electrochemical accelerometer that contains a mechanical oscillating electrolyte-based system and electrochemical transducer that converts the electrolyte flow during oscillations into an electric current. A substantial drawback of this type of accelerometer is that it cannot measure acceleration when the acceleration is constant. This drawback results from the fact that the overall transfer function of the accelerometer, which describes the response of the accelerometer as a function of the frequency of the acceleration, goes to zero when the frequency of the acceleration goes to zero. This behavior can be understood by examining the relationship between the overall accelerometer transfer function and the transfer functions corresponding to the mechanical oscillating system and to the electrochemical transducer, respectively. At zero frequency, the transfer function of the electrochemical transducer becomes a constant, while the transfer function of the mechanical oscillating system goes to zero. Thus, because the overall transfer function of the accelerometer is the product of these two transfer functions, the overall transfer function of the accelerometer goes to zero at zero frequency.
An additional drawback of the linear electrochemical accelerometer described above is the relationship between the value of the low frequency cut-off of the accelerometer and the diameter of the accelerometer. Because the low-frequency cut-off is inversely related to the diameter of the accelerometer, it is impossible to reduce the size of the accelerometer without increasing the low-frequency cut-off, thereby sacrificing some of the performance of the accelerometer. Another drawback of a linear electrochemical accelerometer is that the transfer function of the electromechanical transducer is not an analytical function of the frequency. Thus, additional correction elements must be included in the conditioning electronics in order to obtain uniform sensitivity in a wide frequency range (i.e., a flat acceleration transfer function of the accelerometer). The requirement of additional correction elements increases the self noise of the accelerometer.
Thus, there is an urgent need for an accelerometer that is capable of measuring constant acceleration, and that has wide frequency and dynamic ranges, small size, low power consumption, low weight and low cost.